1998 Ford Explorer Radiator

I was having some issues with overheating and water leakage, and after checking it out my car guy diagnosed the problem as the thermostat. He replaced it, then topped off the water in my radiator (which was maybe 2/3 full) with anti-freeze. Then he capped the radiator and had me start it up. It started fine, but then wouldn't hold a steady idle. It would rev high, then die down to practically nothing and repeat the cycle, at this point he loosened the radiator cap a little. When he did this, the car began to idle normally but water began spewing out from beneath the cover. It was cascading in a steady stream, not just overflowing, but being forcibly pushed out. Then, just before I could turn it off, the water in my overflow tank began bubbling and spitting (through a very small hole in the top) and then erupting out like a volcano. By the time I finally turned off the car there were geysers spewing from both places!
I'm stumped. Can anyone offer some insight?